1. Field of the Invention
The present invention generally relates to a silicon-on-insulator(SOI) device, and more particularly to a body-contacted-SOI(BC-SOI) device in which a field oxide film is not contacted with a buried oxide layer and method of isolating thereof.
2. Description of the Related Art
Due to the fast development in semiconductor device industry, a semiconductor device using the SOI substrate instead of a single crystalline silicon substrate made of bulk silicon, has been proposed. The SOI substrate has a stack structure comprising a base layer as a means for supporting, a buried oxide layer, and a semiconductor layer in which a device is formed later. According to this SOI device, adjoining devices are completely isolated from each other and a reduction of the junction capacitance can be obtained, therefore a low power and high speed device can be manufactured.
FIG. 1 is a cross-sectional view showing a conventional SOI device in which a transistor is formed on an SOI substrate. As shown in the drawing, there is provided the SOI substrate 10 comprising a base layer 1, a buried oxide layer 2 and a semiconductor layer 3. Field oxide films 4a,4b are formed to be contacted with the buried oxide layer 2 in a field region of the semiconductor layer 3. A gate oxide layer 5 and a gate electrode 6 of a transistor 20 are formed in an active region of the semiconductor layer 3 that is defined by those field oxide films 4a,4b. Junction regions 7 such as source and drain regions are formed at active regions of both sides of the gate electrode 6 respectively. Herein, the junction regions 7 are formed to be contacted with the buried oxide layer 2, similar to the field oxide films 4a,4b.
The SOI device as constituted above, may reduce the junction capacitance and the SOI device may be driven with high speed compared to the semiconductor device formed on a silicon substrate since the junction regions are contacted with the buried oxide layer. The SOI device has properties suitable for devices with high speed, however, there is occurred "Floating Body Effect" thereby degrading the operational property.
More particularly, when the transistor formed on the silicon substrate is driven, a body bias is applied to prevent from storing electric charges at a channel region beneath the gate electrode thereby obtaining stabilization in the device operation. However, when the SOI device is driven, since an active region of the semiconductor layer in which the transistor is formed, is floated by the field oxide films and the buried oxide layer, the body bias can not be applied to the active region unless they are equipped with extra device. Consequently, the SOI device has instability of operational property due to the electric charges being stored at the channel region.
Accordingly, to prevent the floating body effect with maintaining those advantages suitable for the devices of high speed and low power, there is proposed the BC-SOI device as disclosed in "Body-Contacted SOI MOSFET structure with fully bulk CMOS compatible layout and process" by Y. H. Koh, J. H. Choi, M. H. Nam and J. W. Yang, IEEE Electron Device Lett., vol. 18, pp. 102.about.104, 1997.
FIG. 2 is a cross-sectional view showing the SOI device. Herein, the same reference numerals are used in the same part of FIG. 2 as in FIG. 1. As shown in the drawing, field oxide films 4c,4d are formed with a depth that is not contacted with the buried oxide layer 2. Furthermore, a diffusion area 8 for well-pick up is provided at a selected portion of the semiconductor layer so that electrical potential of the channel in transistors 20 is controlled. The diffusion area 8 is a doped region with the same conductivity type impurities of an active region of the semiconductor layer 3 i.e. at the body of the transistor. The reference numeral 1 which is not described yet, stands for a base layer, 5 for a gate insulating layer, 6 for a gate electrode, 7 for a junction region and 10 for an SOI substrate.
Also, although not shown in the drawings, a well is provided within the semiconductor layer 3. A semiconductor device is generally a CMOS circuit comprising of NMOS and PMOS, therefore a body of the NMOS may become a P-well and a body of the PMOS may become an N-well.
In the SOI device as described above, since the field oxide film is not contacted with the buried oxide layer, body-floating of the transistors may be prevented. Accordingly, those advantages applicable to the high speed and low power device may be obtained.
However, following drawbacks are also occurred by the BS-SOI device. It is required to thicken the thickness of the field oxide film so as to obtain high quality of isolation property, in other words, to prevent the punch-through problem. When the field oxide film is thick, even though the high quality of isolation property may be obtained, however there is an increase in the Well-resistance since thickness of the semiconductor layer existing between the field oxide film and the buried oxide layer is relatively decreased, therefore instability in the body bias of the transistors is occurred and there is even occurred the floating body effect. While, in case the thickness of the semiconductor layer remained in the lower portion of the field oxide film is increased to reduce the Well-resistance, the punch-through characteristic is degraded due to a decrease of thickness of the field oxide film.
On the other hand, as shown in FIG. 2, the field oxide films 4c,4d are formed with different widths while having the same thickness, and at this time a first distance L.sub.A between a first field oxide film 4c having a first width and the buried oxide layer 2, and a second distance LB between a second field layer 4d having a second width that is relatively narrower than the first width and the buried oxide layer 2 are equal to each other. However, there is a problem of the Well-resistance rather than the punch-through characteristics in the first field oxide region 4c having relatively wider width, and to the contrary there is a problem of the punch-through characteristics rather than the Well-resistance in the second field oxide region 4d having relatively narrower width.
Accordingly, it is difficult to satisfy both Well-resistance and punch-through characteristics in the SOI device since the problem of punch-through characteristics is occurred in the field oxide film having relatively narrower width when the thickness of the field oxide film is decreased, and to the contrary the problem of Well-resistance is occurred in the field oxide film having relatively wider width when the thickness of the field oxide film is increased.